Recent rapid progress of information technology and industry has stimulated development of various information processing systems, and recording methods and recording apparatuses matching such information processing systems have also been developed and commercialized.
Among these recording methods, an inkjet recording method is utilized widely not only in offices but also in so-called home use, because of advantages of possibility of recording on various recording materials, inexpensiveness and compactness of hardware (apparatus), and excellent quietness of recording.
In the inkjet recording method, there are known a method of applying a pressure by a piezoelectric element thereby discharging a liquid droplet, a method of generating a bubble by heat thereby discharging a liquid droplet, a method utilizing ultrasonic wave, and a method of discharging a liquid droplet by attraction with an electrostatic forced. For these inkjet methods, there is employed an aqueous ink, an oily ink or a solid (fusible) ink. Among these, aqueous ink is used principally in consideration of manufacture, handling, smell, safety etc.
A colorant to be employed in such ink for inkjet printing is required to have a high solubility in a solvent, a high density in recording, a satisfactory hue, an excellent fastness to light, heat, air, water and chemicals, a satisfactory fixability to an image receiving material without blotting, an excellent storability in the ink, no toxicity, a high purity, and inexpensive availability. It is however extremely difficult to find a colorant meeting these requirements at a high level.
Various dyes and pigments have already been proposed for use in inkjet recording and are already used in practice, but in fact a colorant satisfying all these requirements have not been found. In already known dyes and pigments such as those represented by color index (C. I.) numbers, it is difficult to obtain a hue required for the ink for inkjet printing and a fastness at the same time. Dyes having a satisfactory hue and a fastness have been investigated and developed for a satisfactory colorant for inkjet printing. However such materials have been found to be significantly inferior in fastness to ozone and to light in comparison with the silver halide-based photograph.
On the other hand, with a recent progress of the inkjet printer toward a higher resolution, it has become possible to obtain so-called photo-like recording of high image quality, and various inkjet recording sheets are being developed, matching such progress in hardware (apparatus).
Properties required for such recording sheet for inkjet printing generally include (1) rapid drying (high ink absorbing speed), (2) appropriate and uniform ink dot diameter (no blotting), (3) satisfactory granularity, (4) high dot circularity, (5) high color density, (6) high saturation (not turbidity in color), (7) satisfactory resistance to water, light and ozone in printed part, (8) high whiteness of recording sheet, (9) satisfactory storability of recording sheet (no yellowing in prolonged storage and no image blotting in prolonged storage (satisfactory blotting resistance in time)), (10) satisfactory dimensional stability without deformation (sufficiently small curl), and (11) satisfactory running ability in hardware.
Also in photo glossy paper used for obtaining so-called photograph-like recording of high quality, there are required a luster, a surface smoothness and a photographic paper-like texture similar to a silver halide-based photograph, in addition to the foregoing properties.
Aiming at improvements in the aforementioned characteristics, an inkjet recording material having an ink receptive layer of a porous structure is recently developed and commercialized. Such inkjet recording material, owing to such porous structure, has a satisfactory ink receiving property (rapid drying property) and a high luster.
For example, JP-A-10-119423 and JP-A-10-217601 propose an inkjet recording material including, on a substrate, an ink receptive layer containing fine inorganic pigment particles and a water-soluble resin and having a high pore rate.
Such inkjet recording material, particularly an inkjet recording material having an ink receptive layer of a porous structure utilizing silica as the inorganic pigment particles, shows a satisfactory ink absorbing property by the structure thereof, thus providing a high ink receiving ability capable of forming an image of a high resolution and also a high luster.
However, trace gases in the air, particularly ozone, may constitute a cause of fading of the recorded image in time. The aforementioned recording material, having the ink receptive layer of the porous structure, is susceptible to fading of the recorded image by ozone gas in the air, because it contains many pores. Therefore, for the aforementioned recording material having the ink receptive layer of the porous structure, a resistance to ozone in the air (ozone resistance) is a very important property.